1. Field of the Invention
The present invention relates to turbocharged internal combustion engines and is directed at maximizing the efficient use of a catalyst and a bypass circuit in an engine capable of multi-fuel stoichiometric and lean-burn combustion strategies and combinations thereof.
2. Description of the Prior Art
Workers in the art of the internal combustion engine at this time are engaged in attempting to reduce exhaust emissions through the use of exhaust purification devices. These devices can reduce engine efficiency by impeding the flow of engine exhaust and in the case of catalysts, often require stoichiometric fuel-air ratios and operation within a narrow temperature range for optimal performance. With catalyst temperature below the optimum range or engine fuel-air ratios excessively rich or lean as typically occurs during engine start and warm up, catalyst performance in breaking down undesirable emissions is highly impeded. Alternatively, exhaust purification devices can overheat by excessive exhaust flow or the ingestion of unburned fuel. The placement of a turbine in the exhaust duct leading to a catalyst for the purpose of driving a turbocharger compressor can absorb heat necessary for catalyst operation.
The clean burning characteristics of fuels such as natural gas offer the possibility of obtaining low exhaust emissions without exhaust after-treatment. This invention is directed at providing an efficient means of controlling exhaust emissions and improving efficiency in multi-fuel turbocharged engines by facilitating the employment of both a supercharged operating mode without exhaust after-treatment for use with a suitable fuel such as natural gas employed in a lean fuel-air mixture, and a naturally aspirated mode utilizing an exhaust catalyzer for stoichiometric fuel air mixtures at low power settings. Another object of the invention is to improve engine efficiency by reducing exhaust backpressure. An additional object of the invention is to facilitate the dual use of either a fuel compatible with supercharged, non-catalyst operation such as natural gas, or a fuel requiring stoichiometric mixtures with a catalyst, such as gasoline. Further objects of the present invention include facilitating rapid heating of an exhaust catalyst during engine start and the avoidance of overheating the catalyst during high power operation.